The present invention relates to a polyester resin composition which is good in moldability and provides molded articles having excellent impact strength and heat resistance. More particularly, the present invention relates to an impact resistant polyester resin composition which has a high crystallization speed and exhibits excellent mold release properties at a mold temperature of not more than about 120.degree. C. in injection molding, and further which provides a molded article having an excellent impact strength and a high heat distortion temperature.
Polyethylene terephthalate is excellent in properties such as mechanical and electrical properties, heat resistance and chemical resistance, and thus is widely used in production of many industrial products such as fibers and films. When used as fibers and films, it is usually subjected to orientation treatment. In the case, however, wherein it is injection molded and used as a molded product in the application of plastics, it is not subjected to such an orientation treatment. Thus, as is well known, various problems arise in the molding of polyethylene terephthalate and physical properties of the molded product. That is, the crystallization speed of polyethylene terephthalate at low temperatures is small; since the crystallization speed is not sufficiently high at a mold temperature of not more than about 120.degree. C. which is usually employed in injection molding of other plastics, the molded product formed is not uniform in mechanical properties, dimensional stability, and shape stability, because of a difference in the crystallization speed between the surface and the inside portion of the molded product. It is therefore quite difficult to produce a molded product which is suitable for practical use.
In order to solve the above problems, various methods have been proposed, including a method using a high temperature mold, a method of adding a crystal nucleating agent and a crystallization-accelerating agent, and a method of blending polyethylene terephthalate with an ethylene terephthalate-based copolymer exhibiting excellent low temperature crystallinity. It has been confirmed that these methods have succeeded in solving the above problems to a considerable extent. Fully crystallized polyethylene terephthalate or a composition containing such polyethylene terephthalate, particularly compounded with a fibrous reinforcing material such as glass fibers exhibits excellent mechanical properties and has a high heat distortion temperature. Thus it is now widely used as an industrial plastic.
However, a disadvantage of a polyethylene terephthalate composition or a reinforced polyethylene terephthalate composition with glass fibers compounded thereto is that the impact strength is low; that is, the toughness is poor. It has therefore been desired to increase the impact strength.
Several methods have been proposed to solve the above problems, i.e., to increase the impact strength. For example, Japanese Patent Application (OPI) Nos. 144452/76, 32045/77, and 17148/83 (the term "OPI" as used herein means a "published unexamined Japanese patent application"), Japanese Patent Publication No. 17151/83, U.S. Pat. Nos. 4,284,540 and 4,461,871 describe that the impact strength of the polyethylene terephthalate composition is increased if copolymers containing a glycidyl group in polyester, such as an ethylene/vinyl acetate/glycidyl acrylate or methacrylate copolymer is compounded. Olefin-based polymers prepared by copolymerization of glycidyl acrylate or methacrylate, i.e., polyolefins containing an epoxy group, when compounded to polyethylene terephthalate (hereinafer abbreviated to "PET"), increases the impact strength of PET, but seriously deteriorates its mold release properties from a mold during the molding process, thereby producing a problem in that the surface luster of the final molded product is poor when a mold of low temperature is used, i.e., the mold temperature is not more than 120.degree. C. A more significant problem is that when polyolefin containing an epoxy group is compounded to PET, and then kneaded and pelletized by the use of an extruder, for example, gel formation occurs to some extent, and, in some cases, a large amount of gel is formed and the operation cannot be performed.
The present inventors, therefore, have discovered that if specific amounts of modified polyolefin or olefinbased elastomer (corresponding to the component (b) of the present invention) and of glycidyl acrylate or methacrylatecopolymerized polyolefin (corresponding to the component (c) of the present invention) are compounded to thermoplastic polyester, the impact strength is greatly increased by the effect of use of the components in combination. Based on this discovery, Tokuzawa et al have filed a Japanese patent application (Japanese Patent Application (OPI) No. 115352/84).
It has been found, however, that with the above composition no molding problem arises when polybutylene terephthalate is used as the thermoplastic polyester, but when PET is used as the thermoplastic polyester, various molding problems arise. That is, if the above composition is used to mold a slightly complicated article, its mold release properties are adversely affected, although this problem is not severe in molding an article having a simple shape, such as a test piece and, therefore, the composition is not suitable for practical use. Moreover, when the mold temperature is low, various problems arise; for example, when the molding temperature is not more than 120.degree. C., not only is the molded product poor in appearance but also even if it is reinforced with glass fibers, for example, the heat distortion temperature is not increased and the heat resistance is poor because of insufficient crystallization, and although the impact resistance in the vicinity of room temperature is increased, the impact resistance at low temperatures between -10.degree. C. and -30.degree. C. is not increased to an expected extent.